Sensor configuration in magnetometer for medical use

ABSTRACT

A magnetometer system for medical use comprises one or more induction coils for detecting a time varying magnetic field. Each coil has a maximum outer diameter of 10 cm or less, and a configuration such that the ratio of the coil&#39;s length to its outer diameter is 0.9 or more, and the ratio of the coil&#39;s inner diameter to its outer diameter is 0.6 or more. Each induction coil comprises a magnetic core. The magnetometer system further comprises a detection circuit coupled to each coil and configured to convert a current or voltage generated in the coil by a time varying magnetic field to an output signal for use to analyse the time varying magnetic field.

The technology described herein relates to methods and apparatus formedical magnetometry, and in particular to methods and apparatus forprocessing a signal from a magnetometer for medical use, such as for useas a cardiac magnetometer.

It can be useful in many medical situations to be able to measuremagnetic fields relating to or produced by the human body for diagnosticpurposes. For example, the heart's magnetic field contains informationthat is not contained in an ECG (Electro-cardiogram), and so a magnetocardiogram scan can provide different and additional diagnosticinformation to a conventional ECG.

Most modern cardiac magnetometers are built using ultra-sensitive SQUID(Superconducting Quantum Interference Device) sensors. However, SQUIDmagnetometers are very expensive to operate as they require cryogeniccooling. Their associated apparatus and vacuum chambers are also bulkypieces of equipment. This limits the suitability of SQUID magnetometersfor use in a medical environment, for example because of cost andportability considerations.

Another known form of magnetometer is an induction coil magnetometer.Induction coil magnetometers have the advantage over SQUID magnetometersthat cryogenic cooling is not necessarily required, they are relativelyinexpensive and easy to manufacture, they can be put to a wide range ofapplications and they have no DC sensitivity.

However, induction coil magnetometers have not been widely adopted formagneto cardiography because magneto cardiography requires low field(<nT), low frequency (<100 Hz) sensing, and common induction coilmagnetometer designs that can achieve such sensitivities are too largeto be practical for use as a cardiac probe.

The Applicants have addressed these problems in their earlierapplication WO2014/006387, which discloses a method and apparatus fordetecting and analysing medically useful magnetic fields that uses aninduction coil or coils of a specific configuration to detect themagnetic field of a subject.

Notwithstanding this, the Applicants believe that there remains scopefor alternative arrangements and improvements to the design and use ofmagnetometers for medical use, and in particular for cardio magneticsensing and/or imaging.

According to a first aspect of the technology described herein there isprovided a magnetometer system for medical use, comprising:

one or more induction coils for detecting a time varying magnetic field,each coil having a maximum outer diameter of 10 cm or less, and aconfiguration such that the ratio of the coil's length to its outerdiameter is 0.9 or more, and the ratio of the coil's inner diameter toits outer diameter is 0.6 or more, wherein each induction coil inembodiments comprises a magnetic core; and

a detection circuit coupled to each coil and configured to convert acurrent or voltage generated in the coil by a time varying magneticfield to an output signal for use to analyse the time varying magneticfield.

According to a second aspect of the technology described herein there isprovided a method of analysing the magnetic field of a region of asubject's body, the method comprising:

using one or more induction coils to detect the time varying magneticfield of a region of a subject's body, each coil having a maximum outerdiameter of 10 cm or less, and a configuration such that the ratio ofthe coil's length to its outer diameter is 0.9 or more, and the ratio ofthe coil's inner diameter to its outer diameter is 0.6 or more, whereineach induction coil in embodiments comprises a magnetic core;

converting a current or voltage generated in each coil by the timevarying magnetic field of the region of a subject's body to an outputsignal; and

using the output signal or signals from the coil or coils to analyse themagnetic field generated by the region of a subject's body.

According to a third aspect of the technology described herein, there isprovided a coil for use to detect the time varying magnetic field of aregion of a subject's body, the coil comprising:

an induction coil having a maximum outer diameter of 10 cm or less, anda configuration such that the ratio of the coil's length to its outerdiameter is 0.9 or more, and the ratio of the coil's inner diameter toits outer diameter is 0.6 or more; and

in embodiments a magnetic core.

The technology described herein provides a method and apparatus fordetecting and analysing magnetic fields that are medically useful orthat could be used as an aid to forming a medical diagnosis, such as themagnetic field of a region of a subject's body (for example of asubject's heart). However, in contrast to existing medical (e.g.cardiac) magnetometer designs, the technology described herein uses aninduction coil or coils (i.e. a coil that is joined to an amplifier atboth ends) of a specific configuration to detect the magnetic field ofthe subject (e.g. of the subject's heart). As will be discussed furtherbelow, the Applicants have found that induction coils having theparticular configuration of the technology described herein can be usedto provide a medical magnetometer that can be portable, relativelyinexpensive, usable at room temperature and without the need formagnetic shielding, and yet can still provide sufficient sensitivity,accuracy and resolution to be medically useful.

By limiting the outer diameter of the coil to 10 cm or less, a coilhaving an overall size that can achieve a spatial resolution that issuitable for medical magnetometry (and in particular for magnetocardiography) is provided.

Setting the ratio of the coil's length to its outer diameter to 0.9 ormore effectively means that the coil is relatively long (along its axis)for its width, e.g. compared to a Brooks coil configuration (for aBrooks coil this ratio is 0.25) and compared with the arrangementdescribed in WO2014/006387 (for which this ratio is 0.69). Setting theratio of the coil's inner diameter to its outer diameter to 0.6 or moremeans that the coil's windings are closely packed in the directionorthogonal to the coil's axis (i.e. have a relatively narrow spread ofradial distances from the coil's axis in the direction orthogonal to thecoil's axis), e.g. compared to a Brooks coil configuration (for whichthis ratio is 0.5) and compared with the arrangement described inWO2014/006387 (for which this ratio is 0.425).

As will be described in more detail below, these requirements for theinduction coil's configuration have been found by the Applicants to makethe coil of the technology described herein particularly sensitive tobiological magnetic fields such as in particular the magnetic field ofthe heart. In particular, the Applicants have found that theserequirements make the coil particularly sensitive where the inductioncoil comprises a soft magnetic core. This is in contrast with thearrangement described in WO2014/006387, which is in effect optimised forinduction coils that do not include a magnetic core (e.g. that areair-cored).

It will be appreciated therefore that the technology described hereinprovides an improved magnetometer system for medical use.

The magnetometer system of the technology described herein can be usedas a system and probe to detect any desired magnetic field produced by asubject (by the human (or animal) body). It is in embodiments used todetect (and analyse) the time varying magnetic field of (or produced by)a region of the subject's body, such as their bladder, abdomen, chest orheart, head or brain, muscle(s), womb or one or more foetuses. Thus itmay be, and is in embodiments, used to detect magnetic fields relatingto the bladder, pregnancy, muscle activity, the brain, or the heart. Invarious embodiments, the magnetometer is used for (and configured for)one or more of: magnetocardiography, magnetoencephalography, analysisand detection of bladder conditions (e.g. overactive bladder), analysisand detection of foetal abnormalities, and detection and analysis ofpre-term labour.

In various particular embodiments the magnetometer is used as a cardiacmagnetometer and to detect and analyse the magnetic field of a subject'sheart.

Thus, according to another aspect of the technology described hereinthere is provided a cardiac magnetometer system for analysing themagnetic field of a subject's heart, comprising:

one or more induction coils for detecting the time varying magneticfield of a subject's heart, each coil having a maximum outer diameter of10 cm or less, and a configuration such that the ratio of the coil'slength to its outer diameter is 0.9 or more, and the ratio of the coil'sinner diameter to its outer diameter is 0.6 or more, wherein eachinduction coil in embodiments comprises a magnetic core; and

a detection circuit coupled to each coil and configured to convert acurrent or voltage generated in the coil by the time varying magneticfield of a subject's heart to an output signal for use to analyse themagnetic field generated by the subject's heart.

According to another aspect of the technology described herein there isprovided a method of analysing the magnetic field of a subject's heart,the method comprising:

using one or more induction coils to detect the time varying magneticfield of a subject's heart, each coil having a maximum outer diameter of10 cm or less, and a configuration such that the ratio of the coil'slength to its outer diameter is 0.9 or more, and the ratio of the coil'sinner diameter to its outer diameter is 0.6 or more, wherein eachinduction coil in embodiments comprises a magnetic core;

converting a current or voltage generated in each coil by the timevarying magnetic field of the subject's heart to an output signal; and

using the output signal or signals from the coil or coils to analyse themagnetic field generated by the subject's heart.

According to another aspect of the technology described herein, there isprovided a coil for use to detect the time varying magnetic field of asubject's heart, the coil comprising:

an induction coil having a maximum outer diameter of 10 cm or less, anda configuration such that the ratio of the coil's length to its outerdiameter is 0.9 or more, and the ratio of the coil's inner diameter toits outer diameter is 0.6 or more; and

in embodiments a magnetic core.

As will be appreciated by those skilled in the art, these aspects of thetechnology described herein can and in embodiments do include any one ormore or all of the optional features of the technology described hereindescribed herein, as appropriate.

The magnetometer system of the technology described herein may comprisea single coil. In this case, the coil may be positioned appropriatelyover a subject (e.g. a subject's chest or other region of the subject'sbody) to take readings from a suitable (single) sampling position forthe region of the subject's body in question. Alternatively, the coilmay be moved over the subject (e.g. the subject's chest) to takereadings from plural different sampling positions in use.

However, in embodiments, the system comprises plural coils, e.g. and inembodiments at least 7, e.g. 7-500 (or more), in embodiments at least16, e.g. 16-500 (or more) coils.

Where the magnetometer system comprises plural coils, some or all of thecoils may be arranged in a two or three dimensional array, e.g. and inembodiments at least 7, in embodiments at least 16, coils arranged in atwo or three dimensional array. In this case, the or each coil array isin embodiments configured such that when positioned appropriately over asubject (e.g. a subject's chest or other region of the subject's body)the coil array can take readings from a suitable set of samplingpositions without the need to further move the array over the subject.

The or each array can have any desired configuration, such as being aregular or irregular array, a hexagonal, rectangular or circular array(e.g. formed of concentric circles), etc.

The number and/or configuration of coils in the or each array is inembodiments selected so as to provide an appropriate number of samplingpoints and/or an appropriate coverage for the region of the subject'sbody in question.

In various embodiments, the coil array is configured to cover a regionof biomagnetic interest, such as the torso or heart. In embodiments,where the magnetometer is used as a cardiac magnetometer to detect andanalyse the magnetic field of a subject's heart, the or each arraycomprises a hexagonal array of at least 7, e.g. 7-50 (or more), inembodiments at least 16, e.g. 16-50 (or more) coils.

An increased number of coils may be provided, e.g. where it is desiredto measure the time-varying magnetic field of a subject's heart with ahigher resolution and/or where it is desired to measure the time-varyingmagnetic field of a region of a subject's body other than the heart,such as in particular the brain. According to various embodiments, theor each array may comprise a hexagonal array of 7, 19, 37, 61, 91, 127,169, 217, 271, 331, 397 (or more) coils.

The magnetometer system may comprise a single layer of coils, or maycomprise plural layers of one or more coils, e.g. and in embodiments2-10 (or more) layers, i.e. one above the other.

In one such embodiment, each coil layer comprises a single coil. In thiscase, then again, the magnetometer may be positioned appropriately overa subject (e.g. a subject's chest or other region of the subject's body)to take readings from a suitable (single) sampling position for theregion of the subject's body in question. Alternatively, themagnetometer may be moved over the subject (e.g. the subject's chest) totake readings from plural different sampling positions in use. However,in various embodiments, one or more or all of the coil layers compriseplural coils, e.g. arranged in a two dimensional array, with one or moreor each array in embodiments arranged as discussed above for the twodimensional array arrangement.

In these embodiments, one or more or each coil in each coil layer may bealigned with one or more or each coil in one or more or all of the otherlayers or otherwise (e.g. anti-aligned), as desired.

Where the magnetometer system comprises plural coils, some or all of thecoils may be connected, e.g. in parallel and/or in series. Connectingplural coils in series will have the effect of increasing the inducedvoltage for a given magnetic field strength. Connecting plural coils inparallel will have the effect of reducing the thermal noise (Johnsonnoise) in the coils. In embodiments, a combination of series andparallel connections is used to optimise the balance of voltage andnoise performance of the coils.

In an embodiment, one or more or each coil in the magnetometer system isarranged in a gradiometer configuration, i.e. where two coils areco-axially aligned (in the direction orthogonal to the plane in whicheach coil's windings are arranged), and where the signal from each ofthe coils is summed, e.g. to provide a measure of a change in themagnetic field in space.

The or each coil in the magnetometer system may comprise any suitablecoil for detecting a time varying magnetic field.

The or each coil is in embodiments configured to be sensitive at leastto magnetic signals between 0.1 Hz and 1 kHz, as this is the frequencyrange of the (majority of the) relevant magnetic signals of the heart.The or each coil may be sensitive magnetic signals outside of thisrange. The or each coil is in embodiments sensitive to magnetic fieldsin the range 10 fT-100 pT.

In the technology described herein, an induction coil or coils (i.e. acoil that is joined to an amplifier at both ends) is used to detect themagnetic field of the subject (e.g. of the subject's heart). Each coilshould (and in embodiments does) comprise one or more windings (e.g.wire) arranged in a coil configuration, e.g. comprising multiple turnsof wire.

Any suitable conductor can be used for the coil's winding(s), such ascopper, aluminium, silver, gold, and alloys and/or platings thereof,etc. However, in various particular embodiments, the coil winding(s)comprises aluminium, in embodiments copper clad aluminium. It would alsobe possible for the coil winding(s) to comprise silver clad aluminium orgold clad aluminium. The use of aluminium has the effect of reducing theweight of the coil, and therefore the weight of the overall magnetometersystem. Furthermore, the addition of copper (or silver or gold) claddingfacilitates production by allowing traditional soldering techniques tobe employed without the need for the aggressive fluxes that aretypically needed for pure aluminium wire.

The total number of turns, N, on the or each coil can be selected asdesired. A particular number of turns for the or each coil is 1000 to10,000, in embodiments 5,000. However, it would be possible for the oreach coil to have more turns than this, e.g. up to 50,000 turns, or upto 100,000 turns, etc.

In various embodiments, each coil comprises multiple layers of turns(i.e. rather than only a single layer of turns). This has the effect ofincreasing the total number of turns, N, and therefore the coil'sinductance, L, e.g. without increasing the coil's length, l. Each coilmay comprise any plural number of layers, such as at least 2 layers,e.g. 2-50 (or more) layers, in embodiments at least 10 layers, e.g.10-50 (or more) layers, in embodiments at least 20 layers, e.g. 20-50(or more) layers, in embodiments around 30 layers of turns. Thus, invarious embodiments, each coil comprises a multi-layer coil.

Each coil may be configured as desired. As discussed above, the coil'slength, l, its outer diameter, D, and the coil's inner diameter, D_(i),are carefully selected in the technology described herein.

Each coil has a maximum outer diameter, D, of 10 cm or less, inembodiments 7 cm or less, in embodiments between 1 and 6 cm, inembodiments between 2 and 5 cm.

Although increasing the outer diameter, D, of the coil in general hasthe effect of increasing the coil's inductance, L, by limiting the outerdiameter of the coil to 10 cm or less, a coil having an overall sizethat can achieve a spatial resolution that is suitable for medicalmagnetometry (and in particular for magneto cardiography) is provided.In particular, this facilitates a medically applicable diagnostic using16 to 50 sampling positions (detection channels) to generate an image.(As discussed above, the data for each sampling position can, e.g., becollected either by using an array of coils, or by using one (orseveral) coils that are moved around the chest to collect the data.)

In addition, limiting the coil's outer diameter to 10 cm or less limitsits weight (and therefore the overall weight of the magnetometersystem), and ensures that the coil is practical and wieldy for use in amagnetometer system.

In the technology described herein, the ratio of the coil's length toits outer diameter, l:D, is 0.9 or more, in embodiments 0.95 or more, inembodiments 1 or more. It would also be possible for the ratio of thecoil's length to its outer diameter, l:D, to be ≥2, ≥3, etc. In variousembodiments, the ratio of the coil's length to its outer diameter, l:D,is also less than 3, in embodiments less than 2.5, in embodiments lessthan 2, in embodiments less than 1.5. Thus, in various embodiments, theratio of the coil's length to its outer diameter, l:D, is in the range0.9 to 3.

Setting the ratio of the coil's length to its outer diameter to at least0.9 effectively means that the coil is relatively long (along its axis)for its width, e.g. compared to a Brooks coil configuration (for aBrooks coil this ratio is 0.25) and compared with the arrangementdescribed in WO2014/006387 (for which this ratio is 0.69). This meansthat the coil can (and in embodiments does) comprise more turns of wirefor a given outer diameter, and thereby increases the coil's inductance,L. This arrangement is also particularly beneficial when the coilcomprise a magnetic core, as will be described in more detail below.

Each coil may have any suitable length, l (i.e. coil winding(s) length).Each coil in embodiments has a length, l, of 10 cm or less, inembodiments between 1 and 10 cm, in embodiments between 3 and 7 cm, inembodiments between 4 and 6 cm. In particular embodiments each coil hasa length, l, of substantially 5 cm.

In this regard, the Applicants have recognised that although increasingthe length, l, (of the windings(s)) of the coil means that the coil cancomprise more turns of wire and can accordingly increase its inductance,L, the benefits of increasing the length (of the windings(s)) of thecoil do not increase linearly as the coil's (winding's) lengthincreases, but instead fall off as the coil's (winding's) lengthincreases. This is because the biological magnetic fields of interestare relatively small, and because the magnetic field strength isinversely proportional to the cube of the distance (1/r³), e.g. from theregion of the subject's body (e.g. heart). This means that turns at the“top” of the coil will experience a different magnetic field strength tothose at the “bottom”. In addition to this, smaller coils are lighter,and are more practical and wieldy for a useful magnetometer system.

As such, the Applicants have found that by limiting the length (of thewindings(s)) of the coil to 10 cm or less, in embodiments between 1 and10 cm, in embodiments between 3 and 7 cm, in embodiments between 4 and 6cm, in particular embodiments to substantially 5 cm, a coil that issufficiently sensitive to biological magnetic fields, and that has anoverall size and weight that can be used in a practical arrangement formedical magnetometry (and in particular for magneto cardiography) isprovided.

In the technology described herein, the ratio of the coil's innerdiameter to its outer diameter (i.e. the ratio of the inner diameter ofthe winding(s) to the outer diameter of the winding(s)), D_(i):D, is 0.6or more. Setting the ratio of the coil's inner diameter to its outerdiameter to 0.6 or more means that the coil's winding(s) are packedrelatively tightly in the direction orthogonal to the core's axis (i.e.have a relatively narrow spread of radial distances from the coil's axisin the direction orthogonal to the coil's axis), e.g. compared to aBrooks coil configuration (for which this ratio is 0.5) and comparedwith the arrangement described in WO2014/006387 (for which this ratio is0.425).

In this regard, the Applicants have recognised that the inductance perturn will not in general be constant for all turns of a (multi-layer)coil. This is because, e.g., the turns of an outer layer of a(multi-layer) coil will have a greater diameter than the turns of aninner layer of the coil, and so the turns of the outer layer willtypically provide a higher inductance per turn. The Applicants havefurthermore recognised that it can be beneficial for the coil to have arelatively consistent inductance per turn (this can, e.g., reducedistortion), and that this can be achieved by ensuring that the ratio ofcoil's inner diameter to its outer diameter, D_(i):D, is as close to oneas possible (i.e. by ensuring that the turns have as narrow a spread ofradial distances from the coil's axis in the direction orthogonal to thecoil's axis as possible). This arrangement is also particularlybeneficial when the coil comprises a magnetic core, as will be describedin more detail below.

On the other hand, as described above, the coil should comprise plurallayers of turns, and increasing the number of layers of turns has theeffect of increasing the coil's inductance (e.g. without increasing thecoil's length, l). However, increasing the number of layers of turnswill decrease the ratio of the coil's inner diameter to its outerdiameter, D_(i):D.

In this regard, the Applicants have found that a particularly beneficialbalance can be found between these competing factors by providing a coilor coils with an inner to outer diameter ratio, D_(i):D, of 0.6:1 ormore, and moreover that this arrangement provides a suitably sensitivemulti-layer coil for which the inductance per turn is relativelyconsistent.

In various particular embodiments, the ratio of the coil's innerdiameter to its outer diameter, D_(i):D, is 0.625 or more, inembodiments 0.65 or more, 0.675 or more, 0.7 or more, 0.725 or more,and/or 0.75 or more. The ratio of the coil's inner diameter to its outerdiameter, D_(i):D, may also be 0.8 or more, or 0.9 or more.

The ratio of the coil's inner diameter to its outer diameter, D_(i):D,should also be (by definition) less than 1. (The upper limit for thisratio is where the coil comprises a single layer of wire.) Thus, theratio of the coil's inner diameter to its outer diameter, D_(i):D, is inembodiments in the range 0.6:1 to ˜1:1. In various embodiments, theratio of the coil's inner diameter to its outer diameter, D_(i):D, isalso less than 0.9, in embodiments less than 0.8.

In various particular embodiments, the or each coil has the followingconfiguration:

4  cm ≤ D ≤ 5  cm; l ≈ 5  cm; and$\frac{D\; i}{D} \approx 0.745$

where D is the outer diameter of the coil, l is length of the coil, andD_(i) is the inner diameter of the coil.

In various other particular embodiments, the or each coil has thefollowing configuration:

4  cm ≤ D ≤ 5  cm; l ≈ 5  cm; and$\frac{D\; i}{D} \approx 0.625$

where D is the outer diameter of the coil, l is length of the coil, andD_(i) is the inner diameter of the coil.

Coils having these proportions have been found to have a particularlyhigh inductance, L, and sensitivity to biological magnetic fields ofinterest.

In various embodiments, a relatively small wire radius is used for thecoil winding(s). This allows the coil to have more layers of turns whilemaintaining a relatively high inner to outer diameter ratio, D_(i):D. Aparticular wire radius is 1 mm or less, in embodiments 0.5 mm or less,in embodiments 0.4 mm or less, in embodiments 0.3 mm or less, inembodiments 0.25 mm or less, in embodiments 0.2 mm or less, inembodiments 0.15 mm or less, in embodiments 0.1 mm or less.

It should be noted here that the use of a relatively small wire radiusgoes against the conventional aim of increasing the wire radius todecrease the coil's resistance and noise. In this regard, the Applicantshave found that when measuring relatively small biological magneticfields, such as the magnetic field of the heart, the increased noise canbe tolerated because of the benefits derived from using a coil with botha relatively high number of turns, N, and a relatively high inner toouter diameter ratio, D_(i):D.

In various embodiments, the coil is as tightly-packed as possible, e.g.in both the direction orthogonal to the core's axis, and in the (axial)direction parallel to the core's axis. In embodiments the coil is asclose to layer-wound as possible, i.e. not scatter wound. However, itwould be possible for the coil to comprise a less tightly wound coil.

The winding density (the ratio of the cross sectional area of thewinding to the cross sectional area of the wire) of the coil is inembodiments in the range 0.5 to 1, in particular embodiments 1. Higherwinding densities facilitate both a relatively high number of turns Nand a relatively high inner to outer diameter ratio, D_(i):D. In otherwords, tighter windings improve the performance of the coil (whereasgaps can introduce losses).

Each coil may have a magnetic core (i.e. the coil windings may be woundaround a magnetic core). In various particular embodiments, a softmagnetic core is used. Providing each coil with a magnetic coreincreases the inductance, L, of the coil. Thus, in various embodimentseach coil comprises a soft magnetic core.

In this regard, the Applicants have found that the requirements for theinduction coil's configuration of the technology described herein makethe coil particularly sensitive where the induction coil comprises amagnetic core. This is in contrast with the arrangement described inWO2014/006387, which is in effect optimised for induction coils that donot include a magnetic core (e.g. that are air-cored).

Any suitable magnetic core material may be used such as a ferromagneticmaterial (e.g. iron), a ferrite or another magnetic material. Inembodiments the core comprises a soft magnetic material such as a softferrite.

In various particular embodiments, the magnetic core is made from amaterial with a high relative permeability, μ_(r), e.g. at least 10, inembodiments at least 1,000 in embodiments at least 10,000, inembodiments at least 100,000. The higher the relative permeability,μ_(r), of the core material, the higher the inductance, L, of the coil.

Suitable high relative permeability materials include, for example,carbon steel (μ_(r)≈100), ferrites such as Nickel Zinc (μ_(r)≈16 to640), Manganese Zinc (μ_(r)>640), pure iron or steel. In embodiments,the magnetic core is made from higher relative permeability materialssuch as magnetic amorphous metal alloys such as Metglas 2714a(μ_(r)>80,000 (e.g. unannealed) to ˜1,000,000 (e.g. annealed)),nano-crystalline material such as FINEMET (μ_(r)>80,000 (e.g.unannealed) to ˜200,000 (e.g. annealed)), nickel-iron alloys such asMu-metal (μ_(r)≈20,000 to 80,000), cobalt iron alloys, and the like.These materials can exhibit very high magnetic permeabilities, but canbe lighter than other magnetic materials such as iron powder. This canbeneficially reduce the overall weight of the magnetometer system.

Each magnetic core in embodiments comprises a cylinder of (soft)magnetic material, and is in embodiments located within the winding(s)of the coil. Thus, the core in embodiments has an outer diameter, D_(c),less than or equal to the inner diameter D_(i) of the coil (winding(s)).

In various particular embodiments, the core has an outer diameter,D_(c), that is close to or equal to the inner diameter, D_(i), of thecoil windings. Thus, the ratio of the core's outer diameter to the innerdiameter of the coil, D_(c):D_(i), is in embodiments 1 or as close to 1as possible, e.g. 0.8 or more, in embodiments 0.9 or more, inembodiments 0.95 or more, in embodiments 0.99 or more. In this regard,the Applicants have found that the core's effect of increasing thecoil's inductance, L, is greater when the core is closer to the coil'swinding(s).

In particular embodiments the outer surface of the core is arranged tobe in contact with the coil winding(s), e.g. for at least part (inembodiments most or all) of its circumference. Arranging for the outersurface of the core to be in contact with the coil winding(s) means thatthe winding(s) are as close as possible to the core, and accordinglythat the core's effect of increasing the coil's inductance, L, is aslarge as possible. It should be noted that this goes against theconventional teaching of providing an air gap between the core and thecoil winding(s) to reduce the possibility of saturation in high magneticfields. In this regard, the Applicants have recognised that the risk ofsaturation is very low in the context of medical magnetometry, sincee.g. biological magnetic field strengths are relatively small.

(It is not, however, necessary for the core to be in direct contact withthe windings. For example, there may be one or more of a (air) gap, aninsulation layer, an adhesive layer or otherwise between the core andthe wire of the winding(s).)

Each core may be solid, e.g. may comprise a solid cylinder (of (soft)magnetic material). However, in various embodiments, each core is atleast partially hollow, e.g. comprises a hollow cylinder (of (soft)magnetic material). In this regard, the Applicants have found that theuse of a hollow core does not significantly reduce the inductance, L, ofthe coil, but can significantly reduce the coil's weight, as well as itscost.

In these embodiments, the percentage of the cross-sectional area of thecore that is hollow (i.e. that is occupied by a hole) can be selected asdesired, e.g. in embodiments 25% or more, in embodiments 50% or more, inembodiments 75% or more, in embodiments 90% or more. Although increasingthe size of the hole in the hollow core can increase the risk ofsaturation, it beneficially reduces the overall cost and weight of thecoil. The hollow core may have any suitable thickness, such as a few mmor less, in embodiments around 1 mm or less.

Equally, the hollow core may be formed in any suitable manner. Invarious particular embodiments, the core comprises one or more sheets ofmagnetic material, e.g. that are formed into a hollow cylinder. The oneor more sheets may be formed into a hollow cylinder, for example, byrolling up one or more sheets, and/or by laminating multiple sheetstogether. In these embodiments, the or each sheet may have any suitablethickness, such as a few mm or less, ≤1 mm, ≤500 μm, ≤100 μm, ≤75 μm,≤50 μm, and/or ≤25 μm. In various particular embodiments, the corecomprises a 35 μm sheet of Metglas 2714a.

The hollow core may comprise a single layer of magnetic material ormultiple layers of magnetic material (e.g. where the sheet of magneticmaterial is rolled around itself multiple times and/or where the corecomprises multiple laminated layers of magnetic material). Where thehollow core comprise multiple layers, any suitable number of layers maybe used, such as 2, 3, 4, 5 or more layers of magnetic material.

Thus, in various embodiments, the core comprises one or more rolledsheets of (soft) magnetic material. This represents a particularlyconvenient, low cost and low weight core arrangement.

As described above, the core is in embodiments made from a materialhaving a high relative permeability, μ_(r) (because increasing thecore's relative permeability increases the coil's inductance). TheApplicants have furthermore recognised that the effective permeability,μ_(e), of the core is determined both by the relative permeability,μ_(r), of the core material and the geometry of the core. In particular,the effective permeability, μ_(e), of the core is determined by theratio of the core's length to its diameter, l_(c):D_(c). Thus, invarious embodiments, the core's outer diameter, D_(c), and its length,l_(c), are carefully selected.

In various particular embodiments, the ratio of the core's length to itsdiameter, l_(c):D_(c), is >1, in embodiments >1.5, in embodiments >2. Itwould also be possible for the ratio of the core's length to itsdiameter, l_(C):D_(c), to be larger than this, e.g. >3. In this regard,the Applicants have recognised that, in general, increasing the ratio ofthe core's length to its diameter, l_(c):D_(c), has the effect ofincreasing the core's effective permeability, μ_(e), and therefore thecoil's inductance, L. This is particularly the case for magneticmaterials with a high relative permeability (e.g. as described above),for which these benefits are highly nonlinear. (Materials with a lowrelative permeability do not benefit as much, if at all, from increasingthis aspect ratio). The ratio of the core's length to its diameter,l_(c):D_(c), is limited by practicality and size considerations. Thus,in various embodiments, the ratio of the core's length to its diameter,l_(c):D_(c), is also <3.

In order to increase the ratio of the core's length to its diameter,l_(c):D_(c), either the core's length, l_(c), can be increased, and/orits diameter, D_(c), can be decreased.

In this regard, it would be possible for the core's length, l_(c), to beless than the length, l, of the coil's winding(s). However, in variousembodiments, the core has a length, l_(c), that is greater than or equalto the length, l, of the coil's winding(s).

Arranging for the core's length, l_(c), to be equal to the length, l, ofthe winding(s) means that the core's length, l_(c), is as long aspossible (and that the core's length to diameter ratio, l_(c):D_(c), isas large as possible) for a given coil winding length, l, withoutincreasing the overall (total) length of the coil.

Arranging for the core's length, l_(c), to be greater than the length,l, of the winding(s) can allow the core's length to diameter ratio,l_(c):D_(c), to be further increased, e.g. at the expense of increasingthe overall length of the coil. In this regard, the Applicants haverecognised that the overall (total) length of the coil (i.e. the lengthincluding the winding(s) and the core) should be (and is in embodiments)30 cm or less, in embodiments between 1 and 10 cm, in embodimentsbetween 3 and 7 cm, in embodiments between 4 and 6 cm, in particularembodiments to substantially 5 cm. This results in a practical andwieldy coil for use in a magnetometer system.

Thus, each core in embodiments has a length, l_(c), of 30 cm or less, inembodiments between 1 and 10 cm, in embodiments between 3 and 7 cm, inembodiments between 4 and 6 cm. In particular embodiments each core hasa length, l_(c), of substantially 5 cm.

It should be noted here that setting the ratio of the coil's length toits outer diameter to 0.9 or more, i.e. so that the coil is relativelylong (along its axis) for its width (as described above), means that thecoil can (and in embodiments does) comprise a magnetic core that has arelatively large length to diameter ratio, l_(c):D_(c), (and accordinglya high effective permeability, μ_(e)), and accordingly that the coilwill have a relatively high inductance, L.

In contrast with increasing the core's length, l_(c), and as will bedescribed in more detail below, decreasing the core's diameter, D_(c),(i.e. to increase the ratio of the core's length to its diameter,l_(c):D_(c)) can lead away from a number of the other particularparameters described above.

In particular, as described above, it is preferable for the coil's innerto outer diameter ratio, D_(i):D, to be relatively large (i.e. 0.6 ormore, and in embodiments as close to 1 as possible), and this isparticularly the case where the coil comprises a magnetic core. This isbecause, as recognised by the Applicants, the magnetic core's effect ofincreasing the coil's inductance is not in general constant for all thecoil's turns. In particular, the turns of the inner layers of the coil,i.e. that are closer to the core than the turns of the outer layers ofthe coil, will experience a significantly larger increase in inductanceper turn due to the presence of the core than the turns of the outerlayers. As such, in order to maximise the effect of the core, the turnsof all of the layers of the coil should be relatively close to the core(and so the coil's inner to outer diameter ratio, D_(i):D, should berelatively large). (For the same reason, and as described above, it ispreferable for the outer diameter of the core, D_(c), to be as close aspossible to the inner diameter D_(i), of the coil.)

However, decreasing the core's outer diameter, D_(c), while maintainingthe outer surface of the core in close proximity with the inner surfaceof the windings, and while maintaining a relatively high coil inner toouter diameter ratio, D_(i):D, (i.e. so that the turns of the coil areall in relative close proximity to the core) could result in a core witha reduced outer diameter D. As described above, reducing the outerdiameter D of the core would, in turn, be expected to reduce the overallinductance, L, of the coil.

In this regard, the Applicants have found that a particularly beneficialbalance between the above described factors is found where the inner toouter diameter ratio, D_(i):D, is 0.6 or more, in embodiments between0.6 and 0.8, in embodiments between 0.6 and 0.7. In particularembodiments, the coil's inner to outer diameter ratio, D_(i):D, issubstantially 0.625. The Applicants have found that for coils thatinclude a core, these proportions provide a particularly highinductance.

Thus, in various particular embodiments, the or each coil has thefollowing configuration:

D ≈ 4  cm; l ≈ 5  cm; and $\frac{D\; i}{D} \approx 0.625$

where D is the outer diameter of the coil, l is length of the coil, andD_(i) is the inner diameter of the coil, and comprises a magnetic corehaving the following configuration:

D_(c)≈2.5 cm; and

l_(c)≈5 cm;

where D_(c) is the outer diameter of the core, and l_(c) is the lengthof the core. Coils having these proportions have been found to have aparticularly high inductance L, and sensitivity to biological magneticfields of interest.

The detection circuit that a coil is coupled to and that is used todetect the output from the coil should, as discussed above, generate anappropriate output signal for analysis from the voltage and/or currentthat is induced in the coil by the magnetic field. Any suitabledetection circuit and arrangement that can do this can be used. Inembodiments the detection circuit converts the voltage or currentgenerated in the coil by the magnetic field into a digital signal forpost processing and averaging.

Where the system includes plural coils, each coil in embodiments has itsown, respective and separate, detection circuit (i.e. there will be asmany detection circuits as there are coils). The output signals from thedetection circuits can then be combined as desired in post processing.

In various embodiments, each detection circuit operates in either avoltage or current sensing mode (in other words, detects and measures asignal generated between the ends of the coil by a time varying magneticfield).

In various embodiments, the voltages produced by the detection circuitare digitised, e.g. for post processing, noise reduction and signalrecovery. Digitisation of the output voltage as early as possible(practical) in the detection setup is preferred to limit amplifiernoise. Thus, in various embodiments, the signal or signals from the oneor more coils is or are digitised, e.g. using one or more digitisers.

The or each digitiser may comprise any suitable digitiser that isoperable to digitise (convert) an analogue signal received from the oneor more coils into a digital signal, e.g. for further processing andaveraging, etc. The digitiser should (and in embodiments does) convert avoltage or current generated in the one or more coils by the magneticfield into a digital signal. In various embodiments, the or eachdigitiser comprises an analogue to digital converter (ADC).

In various embodiments, the magnetometer system comprises a digitisercoupled to each coil and configured to digitise a signal from the coil.Where the system includes plural coils, each coil may have its own,respective and separate, digitiser (i.e. there will be as manydigitisers as there are coils), or some or all of the coils may share adigitiser.

The or each digitiser may be directly connected to the or eachrespective coil, or in embodiments, the or each digitiser may beconnected to the or each respective coil via an amplifier. Thus invarious embodiments, the magnetometer system includes one or moredetection amplifiers, in embodiments in the form of a microphoneamplifier (a low impedance amplifier), connected to one or more or eachcoil, e.g. to the ends of each coil. The or each detection amplifier isin embodiments then connected to a digitiser or digitisers.

The or each amplifier may be configured to have any suitable and desiredamplification level. The or each amplifier may, for example, amplify thesignal (including the noise) received from the or each coil by around1000 times (60 dB) or more.

In various embodiments, the magnetometer system is arranged such thatthe coil and amplifier (that is coupled to the coil) are arrangedtogether in a sensor head or probe which is then joined by a wire to theremaining components of the magnetometer system to allow the sensor head(probe) to be spaced from the remainder of the magnetometer system inuse.

In various embodiments, the (in embodiments digitised) signal or signalsfrom the one or more coils, are averaged over plural periods, e.g. usingaveraging circuitry. The digitised signal or signals may be averagedover plural periods as desired, and the averaging circuitry may compriseany suitable and desired circuitry for averaging the digitised signal orsignals over plural periods.

In an embodiment, a trigger is provided and used for gating (windowing)the signal (i.e. for identifying and dividing the periodic or pseudoperiodic signal into its plural repeating periods). The trigger shouldbe, and in embodiments is, synchronised with the time varying magneticfield of the region of the subject's body. For example, where themagnetometer is used to analyse the magnetic field of a subject's heart,then the signal is in embodiments averaged over a number of heart beats,and an ECG or Pulse Ox trigger from the test subject may be used as adetection trigger for the signal acquisition process.

Thus, in various embodiments, a trigger is used to identify eachrepeating period of the (periodic or pseudo periodic) signal, and thenthe signal is averaged over the plural identified periods.

Other arrangements would, of course, be possible. For example, eachrepeating period of the (periodic or pseudo periodic) signal may beidentified without the use of a trigger, and then the signal may beaveraged over the plural identified periods.

The (in embodiments digitised) signal or signals from the one or morecoils may be filtered, if desired.

In various embodiments, one or more steps are taken to eliminate and/orcompensate for any environmental noise or magnetic field interferencethat may exist in the signal(s) prior to digitisation. Any suitable suchtechniques may be used (e.g. as described in WO2014/006387), although itshould be noted here that the technology described herein does notrequire the use of a magnetically shielded environment.

Other arrangements would, of course, be possible.

It should also be noted that the Applicants have found that heart beatscale sensitivity can be achieved with the technology described hereinwithout using gradient or background noise subtraction (or anyequivalent process to compensate for background noise), although usinggradient or background noise subtraction (or an equivalent process) willallow a useful signal to be produced more quickly.

In various embodiments, any remaining environmental noise (wherepresent) may be reduced and/or removed in post processing.

The system and method of the technology described herein can be used asdesired to analyse the magnetic field, e.g. of the subject's heart. Aheartbeat's waveform and/or information such as a time interval orintervals e.g. between separate heartbeats and/or between certainfeatures within a single heartbeat, and/or a shape or shapes of aheartbeat(s) may be obtained from the output signal or signals. Inembodiments, suitable measurements are taken to allow an appropriatemagnetic scan image of the heart (or other body region of interest) tobe generated, which image can then, e.g., be compared to referenceimages for diagnosis. The technology described herein can be used tocarry out any suitable procedure for imaging the magnetic field of theheart.

In embodiments plural (e.g. 7 to 500 (or more) (e.g. as describedabove)) sampling positions (detection channels) are detected in order togenerate the desired scan image.

The technology described herein accordingly extends to the use of themagnetometer system of the technology described herein for analysing,and in embodiments for imaging, the magnetic field generated by asubject's heart (or other body region), and to a method of analysing,and in embodiments of imaging, the magnetic field generated by asubject's heart (or other body region) comprising using the method orsystem of the technology described herein to analyse, and in embodimentsto image, the magnetic field generated by a subject's heart (or otherregion of the body). The analysis, and in embodiments the generatedinformation and/or image, is in embodiments used for diagnosis of (todiagnose) a medical condition, such as abnormality of the heart, etc.

Thus according to another aspect of the technology described herein,there is provided a method of diagnosing a medical condition,comprising:

using one or more induction coils to detect the time varying magneticfield of a region of a subject's body, each coil having a maximum outerdiameter of 10 cm or less, and a configuration such that the ratio ofthe coil's length to its outer diameter is 0.9 or more, and the ratio ofthe coil's inner diameter to its outer diameter is 0.6 or more, whereineach induction coil in embodiments comprises a magnetic core;

using a detection circuit or circuits coupled to the coil or coils toconvert a current or voltage generated in each coil by the time varyingmagnetic field of the region of the subject's body to a respectiveoutput signal for the coil;

using the output signal or signals from the coil or coils to analyse themagnetic field generated by the region of the subject's body; and

using the analysis of the magnetic field generated by the region of thesubject's body to diagnose said medical condition.

In this aspect of the technology described herein, the output signal orsignals from the coil or coils are in embodiments used to produce animage representative of the magnetic field generated by the region ofthe subject's body, and the method in embodiments then comprisescomparing the image obtained with a reference image or images todiagnose the medical condition. The medical condition is, as discussedabove, in embodiments one of: abnormality of the heart, a bladdercondition, pre-term labour, foetal abnormalities or abnormality of thehead or brain.

As will be appreciated by those skilled in the art, these aspects andembodiments of the technology described herein can and in embodiments doinclude any one or more or all of the optional features of thetechnology described herein described herein, as appropriate.

As will be appreciated from the above, a particular advantage of thetechnology described herein is that it can be used in the normalhospital or surgery or other environment, without the need for magneticshielding. Thus, in various particular embodiments, the methods of thetechnology described herein comprise using the magnetometer system todetect the magnetic field of a subject's heart (or other body region) ina non-magnetically shielded environment (and without the use of magneticshielding). (It would, however, be possible to use the magnetometersystem to detect the magnetic field of a subject's heart (or other bodyregion) in a magnetically shielded environment (and with the use ofmagnetic shielding), if desired.)

Correspondingly, a particular advantage of the technology describedherein is that it can be used without the need for cooling such acryogenic cooling. Thus, in various particular embodiments, the methodsof the technology described herein comprise using the magnetometersystem to detect the magnetic field of a subject's heart (or other bodyregion) without the use of (e.g. cryogenic) cooling. (It would, however,be possible to use the magnetometer system to detect the magnetic fieldof a subject's heart (or other body region) with the use of (e.g.cryogenic) cooling, if desired.)

As will be appreciated by those skilled in the art, all of the aspectsand embodiments of the technology described herein can and inembodiments do include any one or more or all of the optional featuresof the technology described herein, as appropriate.

The methods in accordance with the technology described herein may beimplemented at least partially using software e.g. computer programs. Itwill thus be seen that when viewed from further aspects the technologydescribed herein provides computer software specifically adapted tocarry out the methods herein described when installed on data processingmeans, a computer program element comprising computer software codeportions for performing the methods herein described when the programelement is run on data processing means, and a computer programcomprising code means adapted to perform all the steps of a method or ofthe methods herein described when the program is run on a dataprocessing system. The data processing system may be a microprocessor, aprogrammable FPGA (Field Programmable Gate Array), etc.

The technology described herein also extends to a computer softwarecarrier comprising such software which when used to operate amagnetometer system comprising data processing means causes inconjunction with said data processing means said system to carry out thesteps of the methods of the technology described herein. Such a computersoftware carrier could be a physical storage medium such as a ROM chip,CD ROM or disk, or could be a signal such as an electronic signal overwires, an optical signal or a radio signal such as to a satellite or thelike.

It will further be appreciated that not all steps of the methods of thetechnology described herein need be carried out by computer software andthus from a further broad aspect the technology described hereinprovides computer software and such software installed on a computersoftware carrier for carrying out at least one of the steps of themethods set out herein.

The technology described herein may accordingly suitably be embodied asa computer program product for use with a computer system. Such animplementation may comprise a series of computer readable instructionseither fixed on a tangible medium, such as a non-transitory computerreadable medium, for example, diskette, CD ROM, ROM, or hard disk. Itcould also comprise a series of computer readable instructionstransmittable to a computer system, via a modem or other interfacedevice, over either a tangible medium, including but not limited tooptical or analogue communications lines, or intangibly using wirelesstechniques, including but not limited to microwave, infrared or othertransmission techniques. The series of computer readable instructionsembodies all or part of the functionality previously described herein.

Those skilled in the art will appreciate that such computer readableinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Further, suchinstructions may be stored using any memory technology, present orfuture, including but not limited to, semiconductor, magnetic, oroptical, or transmitted using any communications technology, present orfuture, including but not limited to optical, infrared, or microwave. Itis contemplated that such a computer program product may be distributedas a removable medium with accompanying printed or electronicdocumentation, for example, shrink wrapped software, pre-loaded with acomputer system, for example, on a system ROM or fixed disk, ordistributed from a server or electronic bulletin board over a network,for example, the Internet or World Wide Web.

A number of embodiments of the technology described herein will now bedescribed by way of example only and with reference to the accompanyingdrawings, in which:

FIG. 1 shows schematically the use of an embodiment of the technologydescribed herein for detecting the magnetic field of a subject's heart;

FIGS. 2-5 show further exemplary arrangements of the use of anembodiment of the technology described herein when detecting themagnetic field of a subject's heart;

FIG. 6A shows schematically a coil arrangement in accordance with anembodiment of the technology described herein, and FIG. 6B showsschematically another coil arrangement in accordance with an embodimentof the technology described herein;

FIG. 7 shows a further exemplary arrangement of the use of an embodimentof the technology described herein when detecting the magnetic field ofa subject's heart;

FIG. 8 shows schematically the coil configuration for a Brooks coil;

FIG. 9 illustrates the effect of a coil's outer diameter on the coil'sinductance;

FIG. 10 illustrates the effect of a core's aspect ratio on its effectivepermeability;

FIG. 11 illustrates the effect of a core's aspect ratio on its effectivepermeability;

FIG. 12 shows schematically a single layer coil and a multi-layer coil;

FIG. 13A shows schematically a coil in accordance with an embodiment ofthe technology described herein, and FIG. 13B shows schematicallyanother coil in accordance with another embodiment of the technologydescribed herein; and

FIG. 14 shows schematically a coil in accordance with an embodiment ofthe technology described herein.

Like reference numerals are used for like components where appropriatein the Figures.

FIG. 1 shows schematically the basic arrangement of various embodimentsof a magnetometer system that may be operated in accordance with thetechnology described herein. This magnetometer system is specificallyintended for use as a cardiac magnetometer (for use to detect themagnetic field of a subject's heart). However, the same magnetometerdesign can be used to detect the magnetic field produced by other bodyregions, for example for detecting and diagnosing bladder conditions,pre-term labour, foetal abnormalities and for magnetoencephalography.Thus, although the present embodiment is described with particularreference to cardio-magnetometry, it should be noted that the presentembodiment (and the technology described herein) extends to othermedical uses as well.

The magnetometer system comprises an induction coil 40 coupled to adetection circuit 41 that may contain a number of components.

The detection circuit 41 may comprise a low impedance pre amplifier,such as a microphone amplifier, that is connected to the coil 40, andone or more filters, e.g. one or more a low pass filters, one or morehigh pass filters, one or more band pass filters, and/or one or morenotch filters e.g. to remove line noise (e.g. 50 or 60 Hz andharmonics).

The current output from the coil 40 is processed and converted to avoltage by the detection circuit 41 and provided to an analogue todigital converter (ADC) 42 which digitises the analogue signal from thecoil 40 and provides it to a data acquisition system 43.

A biological signal that is correlated to the heartbeat, e.g. an ECG orPulse-Ox trigger from the test subject may be used as a detectiontrigger for the digital signal acquisition, and the digitised signalover a number of trigger pulses is then binned into appropriate signalbins, and the signal bins overlaid or averaged, by the data acquisitionunit 43. Other arrangements would, however, be possible.

The coil 40 and detection circuit 41 may be arranged such that the coil40 and the preamplifier of the detection circuit 41 are arrangedtogether in a sensor head or probe which is then joined by a wire to aprocessing circuit that comprises the remaining components of thedetection circuit 41. Connecting the sensor head (probe) and theprocessing circuit by wire allows the processing circuit to be spacedfrom the sensor head (probe) in use.

With this magnetometer, the sensor head (probe) will be used as amagnetic probe by placing it in the vicinity of the magnetic fields ofinterest.

FIG. 2 shows an improvement over the FIG. 1 arrangement, which uses inparticular the technique of gradient subtraction to try to compensatefor background noise. (Other techniques could, however, be used). Inthis case, an inverse coil 44 is used to attempt to subtract the effectof the background noise magnetic field from the signal detected by theprobe coil 40. The inverse coil 44 will be equally sensitive to anybackground magnetic field, but only weakly sensitive to the subject'smagnetic field. The inverse coil 44 can be accurately matched to thepickup coil 40 by, for example, using a movable laminated core to tunethe performance of the inverse coil to that of the pickup coil 40.

FIG. 3 shows an alternative gradient subtraction arrangement. In thiscase, both coils 40, 44 have the same orientation, but their respectivesignals are subtracted using a differential amplifier 45. Again, thebest operation is achieved by accurately matching the coils and theperformance of the detection circuits 41. Again, a movable laminatedcore can be used to tune the performance of one coil to match theperformance of the other.

FIG. 4 shows a further embodiment. This circuit operates on the sameprinciple as the arrangement of FIG. 3, but uses a more sophisticatedmethod of field cancellation, and passive coil matching. In particular,a known global magnetic field 44 is introduced to both coils 40, 44 totry to remove background magnetic field interference.

In this circuit, the outputs from the detection circuits 41 are passedthrough respective amplifiers 47, 48, respectively, before beingprovided to the differential amplifier 45. At least one of theamplifiers 47, 48 is tuneable. In use, a known global field 46, such as50 or 60 Hz (and harmonics) line noise, or a signal, such as a 1 kHzsignal, applied by a signal generator 49, is introduced to both coils40, 44.

The presence of a signal on this frequency on the output of thedifferential amplifier 45, which can be observed, for example, using anoscilloscope 50, will then indicate that the coils 40, 44 are notmatched. An amplifier control 51 can then be used to tune the tuneablevoltage controlled amplifier 48 to eliminate the global noise on theoutput of the differential amplifier 45 thereby matching the outputsfrom the two coils appropriately.

In particular embodiments in this arrangement, a known global field of 1kHz or so is applied to both coils, so as to achieve the appropriatecoil matching for the gradient subtraction, but also a filter to remove50 or 60 Hz (and harmonics) noise is applied to the output signal.

FIG. 5 shows a further variation on the FIG. 4 arrangement, but in thiscase using active coil matching. Thus, in this arrangement, the outputsof the coils 40, 44 are again channelled to appropriate detectioncircuits 41, and then to respective amplifiers 47, 48, at least one ofwhich is tuneable. However, the tuneable amplifier 48 is tuned in thisarrangement to remove the common mode noise using a lock in amplifier 52or similar voltage controller that is appropriately coupled to theoutput from the differential amplifier 45 and the signal generator 49.

The above embodiments of the technology described herein showarrangements in which there is a single pickup coil that may be used todetect the magnetic field of the subject's heart. In these arrangements,in order then to make a diagnostic scan of the magnetic fields generatedby a subject's heart, the single pickup coil can be moved appropriatelyover the subject's chest to take readings at appropriate spatialpositions over the subject's chest. The readings can then be collectedand used to compile appropriate magnetic field scans of the subject'sheart.

It would also be possible to arrange a plurality of coil and detectioncircuit arrangements, e.g. of the form shown in FIG. 1, in an array, andto then use such an array to take measurements of the magnetic fieldgenerated by a subject's heart. In this case, the array of coils couldbe used to take readings from plural positions over a subject's chestsimultaneously, thereby, e.g., avoiding or reducing the need to takereadings using the same coil at different positions over the subject'schest.

FIGS. 6A and 6B show suitable coil array arrangements that have an array60 of 16 detection coils 61, which may be then placed over a subject'schest to measure the magnetic field of a subject's heart at 16 samplingpositions over the subject's chest. FIG. 6A shows a regular rectangulararray and FIG. 6B shows a regular hexagonal array. In these cases, eachcoil 61 of the array 60 should be configured as described above andconnected to its own respective detection circuit (i.e. each individualcoil 61 will be arranged and have a detection circuit connected to it asshown in FIG. 1). The output signals from the respective coils 61 canthen be combined and used appropriately to generate a magnetic scan ofthe subject's heart.

Other array arrangements could be used, if desired, such as circulararrays, irregular arrays, etc.

More (or less) coils could be provided in the array, e.g. up to 50coils, or more than 50 coils. For example, where it is desired tomeasure the magnetic field of a different region of a subject's body(i.e. other than the heart), then an increased number of coils may beprovided so as to provide an appropriate number of sampling points andan appropriate spatial coverage for the region of the subject's body inquestion.

It would also be possible in this arrangement to use some of the coils61 to detect the background magnetic field for the purposes ofbackground noise subtraction, rather than for detecting the wanted fieldof the subject's heart. For example, the outer coils 62 of the arraycould be used as background field detectors, with the signals detectedby those coils then being subtracted appropriately from the signalsdetected by the remaining coils of the array. Other arrangements forbackground noise subtraction would, of course, be possible.

It would also be possible to have multiple layers of arrays of the formshown in FIG. 6, if desired. In this case, there could, for example, betwo such arrays, one on top of each other, with the array that is closerto the subject's chest being used to detect the magnetic field generatedby the subject's heart, and the array that is further away being usedfor the purposes of background noise detection.

To measure the magnetic fields generated by the heart, the abovearrangements can be used to compile magnetic field scans of a subject'sheart by collecting magnetic field measurements at intervals over thesubject's chest. False colour images, for example, can then be compiledfor any section of the heartbeat, and the scans then used, for exampleby comparison with known reference images, to diagnose various cardiacconditions. Moreover this can be done for significantly lower costs bothin terms of installation and on-going running costs, than existingcardiac magnetometry devices.

FIG. 7 shows an exemplary arrangement of the magnetometer as it isenvisaged it may be used in a hospital, for example. The magnetometer 30is a portable device that may be wheeled to a patient's bedside 31 whereit is then used to take a scan of the patient's heart (e.g.). There isno need for any magnetic shielding, cryogenic cooling, etc. Themagnetometer 30 can be used in the normal ward environment. (Magneticshielding and/or cooling could, however, be provided if desired.)

In the technology described herein, each coil's 61 length, l, its outerdiameter, D, and its inner diameter, D_(i), are carefully selected inorder to improve the coil's 61 sensitivity to bio-magnetic fields.

In its simplest form, an induction coil is an electronic component thatresponds to changes in a magnetic field by producing an electromotiveforce (EMF, or voltage difference) in opposition (due to Lenz's law) tothe field that produced this force. From this induced potentialdifference (voltage), a current will flow through the coil.

It has been shown mathematically that the maximum possible inductance ofa coil with an air core for a given length of wire is the Brooks coil.

FIG. 8 illustrates the design of the conventional Brooks coil. Here thewinding cross-section is square and the overall diameter of the coil hasa width of 4 times one of the sides of the square. The inductance L forthe Brooks coils is given by the equation:

L≅0.02591hN ²μ₀ H

where h is the height or length of one side of the square winding crosssection, N is the total number of turns, μ₀ is the permeability of freespace, and H is the magnetic field strength. This can also be expressedin terms of the mean winding radius (r_(mean)) of the coil as follows:

L≅0.016994r _(mean) N ²μ₀ H

H can also be expressed as BA which represents the magnetic flux densityB multiplied by the cross-sectional area A of the coil:

H=BA=Bπr _(mean) ²

It can be seen from these equations that to increase the inductance L ofan air-core coil, either the radius of the coil r_(mean) or the numberof turns N must be increased. However, both of these add to theelectrical resistance of the coil.

FIG. 9 illustrates the effect of a coil's outer diameter, D, on thecoil's inductance, L. In FIG. 9, the solid (lower) line shows the changein measured inductance L with diameter D of a coil that does not have amagnetic core, while the dashed (upper) line shows the change inmeasured inductance L with diameter D of a coil that has a soft magneticcore. Each coil measured in FIG. 9 had a fixed number of (30) turns.FIG. 9 shows that the presence of a soft magnetic core improves theinductance of the coil. Moreover, FIG. 9 shows that, for a fixed numberof turns, the inductance L of a (single-layer) coil increases with itsdiameter (and cross-sectional area), allowing it to cut more magneticflux lines.

Most conventional coil designs are based upon the Brooks coil andwinding cross-sections. Air core coils are commonly used because they donot saturate easily and experience low losses, particularly at higherfrequencies. Often air-gaps are deliberately introduced to certaininductors to reduce core saturation. Inner-to-outer diameter ratios aretypically small. Large gauges of wire are chosen to reduceresistance/noise which results in physically large and heavy coils.

Another way to increase the sensitivity of an induction coil withoutincreasing resistance is to introduce a soft-magnetic (ferrous) material(core) into the centre of the coil.

Ferrous cores are materials possessing high magnetic permeability andcan be used to guide and confine magnetic fields. When introduced to aninduction coil, they can greatly enhance the magnetic field strength.Ferrous cores act as flux concentrators within the coil which drawmagnetic field lines to themselves, greatly increasing the inductance ofthe coil.

The inductance of a single-layer coil with a ferrous core is given bythe following formula:

L=μ _(e)μ₀ N ² BA

Here, μ_(e) refers to the effective permeability of the ferrous materialin the centre (which is equal to 1 in the case of air).

FIG. 9 shows the increase in inductance for each coil with theintroduction of a soft magnetic material core. It can be seen that a 10mm diameter coil with a core has an equivalent induction to a 50 mmdiameter coil without a core despite having 25× less cross-sectionalarea (and significantly, lower resistance).

However magnetic cores are not without their downsides as they canintroduce losses primarily through hysteresis and eddy currents. Highpermeability alone is not a sufficient for a material to be selected asa magnetic core. Generally speaking materials with low coercivity arepreferred as it allows them to respond to changing (AC) fields withlower losses (materials with high coercivity can be considered permanentmagnets).

A number of coils that use a magnetic core and that are configured inaccordance with embodiments will now be discussed.

Increasing the inductance of a coil has a number of positive effects,including increasing the coil's sensitivity to magnetic fields, andincreasing the time constant of the voltage rise time, thereby shiftingthe frequency response of the coil to lower frequencies (which are moretypical of biological signals) and acting as a choke forhigher-frequency sources of noise.

According to various embodiments, amorphous metallic alloys (sometimesreferred to as metallic glasses or glassy metals) cores are used, e.g.in place of conventional pressed iron powder cores. These materialsdiffer from traditional metallic materials and alloys in that they havehighly disordered atomic structures instead of conventional crystallineor poly-crystalline lattices, and as such have a number or uniqueproperties.

By alloying with certain magnetic materials such as iron, cobalt, andnickel, very high magnetic permeability and susceptibility materials arepossible, such as Metglas 2714a or FINEMET. Their high resistancereduces eddy current losses when subjected to alternating magneticfields; their low coercivity also reduces losses.

As such, a core of, for example, Metglas 2714a, nano-crystallinematerials (i.e. polycrystalline materials with very small grain sizes,the space between which are filled with amorphous material), or MuMetal,may be used.

The Applicants have recognised that the effective permeability (μ_(e))of the magnetic core will depend both of the relative permeability(μ_(r)) of the magnetic material, and also on the geometry of the core.In particular, the effective permeability (μ_(e)) depends on the core'sgeometry-dependent demagnetizing factor N_(demag):

${\mu_{e} = \frac{\mu_{r}}{1 + {N_{demag} \cdot \left( {\mu_{r} - 1} \right)}}},{{where}\text{:}}$$N_{demag} \cong {\frac{D_{C}^{2}}{l_{C}^{2}} \cdot {\left( {{\ln \frac{2\; l_{C}}{D_{C}}} - 1} \right).}}$

Here, D_(c) and l_(c) are the diameter and length of the core.

For sufficiently large relative permeabilities, the effective corepermeability is almost independent of material properties because thisformula simplifies to:

$\mu_{e} \approx \frac{1}{N_{demag}}$

FIG. 10 illustrates the effect of a core's aspect ratio on its effectivepermeability, μ_(e). As can be seen from FIG. 10, a material with arelative permeability of 10,000 may have an effective permeability of 4when the length and diameter of the core are equal, or an effectivepermeability of >1000 when the core is 100 times longer than it is wide.

This can be seen more readily by re-plotting the data from air-corecoils (depicted in FIG. 9) and scaling the inductance to be relative tothat of the coil without a core present. This is shown in FIG. 11. Here,with the addition of a fixed core length (50.8 mm), but variable corediameter (x-axis), drastically different values of inductance are seenfor the same electrical resistance coil. Coils with a core diameter of 5mm (10:1 aspect ratio) exhibit a ˜17 times increase in measuredinductance compared to only a ˜2.5 times increase for coils with a 50 mmdiameter core (˜1:1 aspect ratio).

Taking this insight to its logical extreme, the most sensitive coilshave a high permeability core, many turns, present a largecross-sectional area and maintain a high aspect ratio. Unfortunatelyphysical constraints mean that it is not practical to produce a coillarger than a certain length and so a compromise must be struck.

In this regard, the outer diameter of the coil, D, should be limited toaround 10 cm or less, in order to provide a coil having an overall sizethat can achieve a spatial resolution that is suitable for medicalmagnetometry (and in particular for magneto cardiography).

The ratio of the coil's length to its outer diameter should berelatively large (i.e. 0.9 or more), so that the coil is relatively long(along its axis) for its width. This means that the coil can comprise amagnetic core that has a relatively large length to diameter ratio,l_(c):D_(c), (and accordingly a high effective permeability, μ_(e)), andaccordingly that the coil will have a relatively high inductance, L.

However, the magnetic field strength falls off proportionally with 1/r³,so turns that are twice as far away from the source of the magneticfield experience a field strength reduced by a factor of 8. Thus, forexample, turns 10 cm from the magnetic field source (e.g. the top ofheart, or middle, etc.) will experience a magnetic field strength of12.5% of the strength experienced at 5 cm from the source; turns 15 cmfrom the source will experience a magnetic field strength of 3.7% thestrength experienced at 5 cm, and 29% of the strength experienced at 10cm; and turns 20 cm from the source will experience a magnetic fieldstrength of 1.56% of the strength experienced at 5 cm, 12.5% of thestrength experienced at 10 cm, and 42% of the strength experienced at 15cm.

This means that turns at the top and bottom of coil experience verydifferent field strengths. This in turn means that there is no benefitin designing a very long coil despite improvements due to the aspectratio. From these consideration, it was determined that the optimum coillength, l, is ˜50 mm. Beyond this length, the field of the heart weakensand diverges significantly, and the magnetometer device becomes lesspractical and unwieldy.

Furthermore, the Applicants have recognised that the ratio of the coil'sinner diameter to its outer diameter (i.e. the ratio of the innerdiameter of the winding(s) to the outer diameter of the winding(s)),D_(i):D, should be relatively large, i.e. 0.6 or more. This means thatthe coil's winding(s) are packed relatively tightly in the directionorthogonal to the core's axis (i.e. have a relatively narrow spread ofradial distances from the coil's axis in the direction orthogonal to thecoil's axis). This in turns means that the turns of each layer of thecoil will be relatively close to the core.

In addition, arranging the outer surface of the core to be in contactwith the coil winding(s), means that the winding(s) are as close aspossible to the core. Turns of the coil in direct contact or in closeproximity with the core receive a large boost to their measuredinductance values, but the effect can be almost negligible for the outerturns.

On the other hand, each coil should comprise plural layers of turns,since increasing the number of layers of turns has the effect ofincreasing the coil's inductance (e.g. without increasing the coil'slength, l). However, increasing the number of layers of turns willdecrease the ratio of the coil's inner diameter to its outer diameter,D_(i):D.

This can be addressed to some degree by using a wire with a smallergauge and hence cross-sectional area, and so more turns (and thusincreased wire length) can be added in the same volume, or an identicalnumber of turns can be placed in close proximity to the core. This isillustrated by FIG. 12.

This change comes at the expense of increased resistance as it isproportional to cross-sectional area (A):

$R = \frac{\rho \; l}{A}$

However, if the resistance becomes too high, then the Johnson-Nyquistnoise can becomes a problem (this can increase the number of cycleaverages needed and prolong scan times, or increase the size of thesmallest detectable feature), and the amplification electronics need tobe suitably modified to ensure sufficient current flow. Though thetemperature can be reduced to minimize noise levels, cryogenicrefrigerants (such as liquid nitrogen or helium) are not practical, e.g.in terms of cost and safe containment.

The above factors must be carefully balanced when designing coils foruse in medical magnetometry. In this regard, the Applicants have foundthat a particularly balance between the above described competingfactors can be found by providing a coil or coils with the followingconfiguration:

D ≈ 4.7  cm; l ≈ 5  cm; and $\frac{D\; i}{D} \approx 0.745$

where D is the outer diameter of the coil, l is length of the coil, andD_(i) is the inner diameter of the coil. This arrangement is depicted inFIG. 13B. Coils having these proportions have been found to have arelatively high inductance, L.

In various other particular embodiments, the or each coil has thefollowing configuration:

D ≈ 4  cm; l ≈ 5  cm; and $\frac{D\; i}{D} \approx 0.625$

where D is the outer diameter of the coil, l is length of the coil, andD_(i) is the inner diameter of the coil. This arrangement is depicted inFIG. 13A. Coils having these proportions have been found to have an evenhigher inductance, L.

The Applicants have also found that it is not necessary to have acompletely solid core, so long as the overall dimensions of the core aremaintained. Indeed, a thin strip or ribbon (e.g. <35 μm thick) ofMetglas 2714a foil rolled into a hollow cylinder (or e.g. a laminatedstack of layers formed into a hollow cylinder) and placed into a coilcan yield a similar (or even greater) increase in coil inductance to aferrite rod of the same overall dimensions because of its high relativepermeability (μ_(r)). This results in significant reductions in bothmaterial costs and coil weight. Similar benefits can be obtained usingplural laminated layers of foil.

The Applicants have also found that, for these high aspect ratio coreshapes, it is important to place the core material in direct contactwith (or close to) the windings. This minimizes the potential forleakage inductance and partially serves as a choke to filter outundesired high-frequency noise. As such, the coils may compriseself-supporting bonded coils (i.e. instead of winding onto a bobbin andintroducing an “air” gap between core and wire).

The wire used is 0.25 mm copper or copper clad aluminium. By reducingthe wire gauge and increasing the length of the coil, many more turnsare able to be wound, significantly increasing the inductance of thecoil. By using copper clad aluminium, the weight of the coil issignificantly reduced (e.g. compared to copper). If the weight of thecoil grows too large, then the cost and engineering challenge of safelyfixing them above the patient increases. Copper clad aluminium can offersignificant (>50%) weight reductions at the price of increasedresistance.

The coils according to various embodiments are around 10 times moreresistive, so exhibit around 3 times more thermal noise than the coilsdescribed in WO2014/006387. The inductance however is around 11 timeshigher, so the signal to noise ratio is improved by a factor of morethan 3.

Although as shown in FIG. 13, the coil's 61 core 70 may have the samelength as the winding 71, as illustrated by in FIG. 14, it would also bepossible for the core 70 to be longer than the length of the winding 71.This can increase the aspect ratio of the core 70, and so increase itseffective permeability. (It should be noted that FIG. 14 is forillustrative purposes only, and is not to scale.)

The presence of a magnetic core significantly increases the inductanceof the coils. The use of air cored coils having the configurationdescribed herein to detect biological magnetic fields of interest wouldnecessitate significantly increased scan times in order to obtain thesame signal to noise ratio.

It can be seen from the above that the technology described herein, inits embodiments at least, provides a magnetic imaging device that can bedeployed effectively from both a medical and cost perspective in a widerange of clinical environments, e.g. for use when detecting magneticfields generated by the heart. The magnetometer is, in particular,advantageous in terms of its cost, its practicality for use in clinicalenvironments, and its ability to be rapidly deployed for near patientdiagnosis and for a wide range of applications. It is non-contact, worksthrough clothing, fast, compact and portable and affordable. An imagecan be recovered with high resolution after a minute of signal recordingand absolute “single beat” sensitivity is potentially possible. Withappropriate data treatment, slight patient motion will not significantlydegrade the image.

This is achieved, in embodiments of the technology described herein atleast, by using an improved design of detection coil that has aparticular configuration and that is configured to detect the timevarying magnetic of the (e.g.) heart.

1. A magnetometer system for medical use, comprising: one or moreinduction coils for detecting a time varying magnetic field, each coilhaving a maximum outer diameter of 10 cm or less, and a configurationsuch that the ratio of the coil's length to its outer diameter is 0.9 ormore, and the ratio of the coil's inner diameter to its outer diameteris 0.6 or more, wherein each induction coil comprises a magnetic core;and a detection circuit coupled to each coil and configured to convert acurrent or voltage generated in the coil by a time varying magneticfield to an output signal for use to analyse the time varying magneticfield.
 2. The magnetometer system of claim 1, comprising pluralinduction coils arranged in one or more two or three dimensional arrays.3. The magnetometer system of claim 1, wherein each induction coilcomprises plural layers of turns.
 4. The magnetometer system of claim 1,wherein each induction coil has a winding length of 10 cm or less. 5.The magnetometer system of claim 1, wherein each induction coil has aconfiguration such that the ratio of the coil's length to its outerdiameter is in the range 0.9 to 3, and the ratio of the coil's innerdiameter to its outer diameter is in the range 0.6 to
 1. 6. Themagnetometer system of claim 1, wherein each induction coil has aconfiguration such that the ratio of the coil's length to its outerdiameter is in the range 1 to 1.5.
 7. The magnetometer system of claim1, wherein each induction coil comprises wire having a radius less than0.2 mm.
 8. The magnetometer system of claim 1, wherein each magneticcore comprises a material with a relative permeability, μ_(r), of atleast
 1000. 9. The magnetometer system of claim 1, wherein each magneticcore comprises a magnetic amorphous metal alloy, a nano-crystallinematerial, a nickel-iron alloy or a cobalt-iron alloy.
 10. Themagnetometer system of claim 1, wherein the ratio of each core's outerdiameter to the coil's inner diameter, D_(c):D_(i), is 0.8 or more. 11.The magnetometer system of claim 10, wherein the ratio of each core'souter diameter to the coil's inner diameter, D_(c):D_(i), is 0.9 ormore.
 12. The magnetometer system of claim 1, wherein the ratio of eachcore's length to its diameter, l_(c):D_(c), is at least
 1. 13. Themagnetometer system of claim 1, wherein each core is hollow.
 14. Acardiac magnetometer system for analysing the magnetic field of a regionof a subject's body, comprising the magnetometer system of claim
 1. 15.A coil for use to detect the time varying magnetic field of a region ofa subject's body, the coil comprising: an induction coil having amaximum outer diameter of 10 cm or less, and a configuration such thatthe ratio of the coil's length to its outer diameter is 0.9 or more, andthe ratio of the coil's inner diameter to its outer diameter is 0.6 ormore; and a magnetic core.
 16. A method of analysing the magnetic fieldof a region of a subject's body, the method comprising: using one ormore induction coils to detect the time varying magnetic field of aregion of a subject's body, each coil having a maximum outer diameter of10 cm or less, and a configuration such that the ratio of the coil'slength to its outer diameter is 0.9 or more, and the ratio of the coil'sinner diameter to its outer diameter is 0.6 or more, wherein eachinduction coil comprises a magnetic core; converting a current orvoltage generated in each coil by the time varying magnetic field of theregion of a subject's body to an output signal; and using the outputsignal or signals from the coil or coils to analyse the magnetic fieldgenerated by the region of a subject's body.
 17. The method of claim 16,comprising using the induction coils to detect the magnetic field of aregion of a subject's body in a non-magnetically shielded environment.18. The method of claim 16, wherein the region of the subject's bodywhose magnetic field is being analysed comprises one of: the abdomen,bladder, heart, head, brain, chest, womb, one or more foetuses, or amuscle.
 19. A method of analysing the magnetic field of a subject'sheart, the method comprising: using the method of claim 16 to analysethe time varying magnetic field of a subject's heart.
 20. The use of themagnetometer system of claim 1 for analysing the magnetic fieldgenerated by a region of a subject's body.